<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <div id="app">
        {{a}}
    </div>
    <script src="../dist/vue.js"></script>
    <script>
         new Vue({
             el: '#app',
            data: function () {
                return {
                    a:1
                }
            },
            mounted(){
                setTimeout(() => {
                    this.a = 2
                }, 1000);
            },
        })
    </script>
</body>
</html>